In high temperature gasification processes, a hot partial oxidation gas is produced from hydrocarbonaceous fuels, for example coal, oils, hydrocarbon wastes, and the like. In these processes, the hydrocarbonaceous fuels are reacted with a reactive oxygen-containing gas, such as air or oxygen, in a gasification reactor to obtain the hot partial oxidation gas.
The term "hydrocarbonaceous" as used herein to describe various suitable feedstocks is intended to include gaseous, liquid, and solid hydrocarbons, carbonaceous materials, and mixtures thereof. In fact, substantially any combustible carbon-containing organic material, or slurries thereof, may be included within the definition of the term "hydrocarbonaceous". Solid, gaseous, and liquid feeds may be mixed and used simultaneously; and these may include paraffinic, olefinic, acetylenic, naphthenic, and aromatic compounds in any proportion. Also included within the definition of the term "hydrocarbonaceous" are oxygenated hydrocarbonaceous organic materials including carbohydrates, cellulosic materials, aldehydes, organic acids, alcohols, ketones, oxygenated fuel oil, waste liquids and by-products from chemical processes containing oxygenated hydrocarbonaceous organic materials, and mixtures thereof.
The term "liquid hydrocarbon," as used herein to describe suitable liquid feedstocks, is intended to include various materials, such as liquefied petroleum gas, petroleum distillates and residue, gasoline, naphtha, kerosene, crude petroleum, asphalt, gas oil, residual oil, tar-sand oil and shale oil, coal derived oil, aromatic hydrocarbons (such as benzene, toluene, xylene fractions), coal tar, cycle gas oil from fluid-catalytic-cracking operations, furfural extract of coker gas oil, and mixtures thereof.
"Gaseous hydrocarbons," as used herein to describe suitable gaseous feedstocks, include methane, ethane, propane, butane, pentane, natural gas, coke-oven gas, refinery gas, acetylene tail gas, ethylene off-gas, and mixtures thereof.
"Solid hydrocarbon fuels," as used herein to describe suitable solid feedstocks, include, coal in the form of anthracite, bituminous, subbituminous; lignite; coke; residue derived from coal liquefaction; peat; oil shale; tar sands; petroleum coke; pitch; particulate carbon (soot or ash); solid carbon-containing waste materials, such as sewage; and mixtures thereof. Certain types of hydrocarbonaceous fuels, in particular coal and petroleum coke, generate high levels of ash and molten slag.
In the reaction zone of a gasification reactor, the hydrocarbonaceous fuel is contacted with a free-oxygen containing gas, optionally in the presence of a temperature moderator. In the reaction zone, the contents will commonly reach temperatures in the range of about 1,700.degree. F. (930.degree. C.) to about 3,000.degree. F. (1650.degree. C.), and more typically in the range of about 2,000.degree. F. (1100.degree. C.) to about 2,800.degree. F. (1540.degree. C.). Pressure will typically be in the range of about 1 atmosphere (100 Kpa) to about 250 atmospheres (25,000 KPa), and more typically in the range of about 15 atmospheres (1500 Kpa) to about 150 atmospheres (1500 KPa).
In a typical gasification process, the hot partial oxidation gas will substantially comprise H2, CO, and at least one gas from the group H.sub.2 O, CO.sub.2, H.sub.2 S, COS, NH.sub.3, N.sub.2, and Ar. Particulate carbon, ash, and/or molten slag typically containing species such as SiO.sub.2, Al.sub.2 O.sub.3, and the oxides and oxysulfides of metals such as Fe and Ca are commonly produced by well known partial oxidation processes in the reaction zone of a free-flow, down-flowing vertical refractory lined steel pressure vessel. An example of such a process and pressure vessel are shown and described in U.S. Pat. No. 2,818,326, which is hereby incorporated by reference.
Thermocouples are commonly used for measuring temperature in these high temperature processes, including the temperature in the gasification reactor. Thermocouples are pairs of wires of dissimilar metals which are connected at both ends. The content of the wires must be sufficiently dissimilar to allow for a difference in electrical potential between them. Except for the junction at the end of the thermocouple, the two wires are electrically insulated from each other in a protective sheath. The electrical insulation is commonly provided by the protective sheath which consists of a temperature resistant electrically insulating material having two non-intersecting holes extending axially through a portion of the length of the sheath, wherein the thermocouple wires are run through the holes and wherein the holes intersect one another only at one point. Typical protective sheath materials include high temperature, high purity ceramics, such as alumina. The holes may be formed by casting the refractory material around the thermocouple wires and sensor.
The basis of operation of a thermocouple is that an electrical potential that exists between connecting metals varies with temperature. The electrical potential is compared to the potential of a real or an artificial standard that represents the same metals at a standard temperature, and the difference in temperature is measured by a voltage measuring instrument placed in the thermocouple circuit or alternatively by a voltage; measuring instrument that is sent signals by a transmitter placed in the thermocouple circuit. The choice of dissimilar metals used for the thermocouple will vary depending on, among other things, the expected temperature range to be measured. For instance, one type of thermocouple commonly employed under the conditions present in a gasification reactor has one wire that contains platinum and about 30% rhodium and a second wire that contains platinum and about 6% rhodium. For a gasification reactor, type B, type R, and type S platinum/rhodium thermocouples are useful.
The thermocouples have very short lifespans in the environment present in a gasification process, particularly in the environment present in the gasification reactor. The relatively short lifespan is due in part to the corrosive atmosphere that prevails during the operation of the gasification reactor. An unprotected thermocouple left in this environment is quickly attacked and rendered useless. Such attack can be most severe when the thermocouple comes into contact with molten slag present in the reactor. Such a thermocouple may be rendered inoperable in minutes.
To alleviate this problem, thermocouples are commonly inserted into a refractory thermowells mounted along the outer wall of a gasification reactor. The refractory thermowells would include barriers of chromia-magnesia, chromia, or similar slag resistant materials, and may incorporate other refractory and non-refractory materials such as Al.sub.2 O.sub.3, MgO, sapphire, molybdenum, and stainless steel. These refractory thermowells do not make a complete barrier to the atmosphere inside the gasifier. The refractory thermowell does not stand up to pressure, and does not stand up to stress. It is simply a semipermeable mass transfer barrier that protects the thermowell from slag, direct flame, and some hot gases.
When used in a gasification reactor, the thermowell may be introduced by passing it through an opening in the outer wall of the reactor pressure vessel. The thermowell may then pass through a corresponding opening in a refractory material, or series of refractory materials, commonly used to line the inner surface of the reactor pressure vessel. The thermowell may extend into the open space of the reactor or more typically it may be set back at a slight distance from the interior of the reactor.
Unfortunately, positioning the thermocouple inside a thermowell has not provided a complete solution. Over time, molten slag will breach the thermowell. The breach is commonly due to the effects of erosion and corrosion as well as thermal and/or mechanical stress. The breach, typically small initially, allows molten slag to enter the thermowell where it can come in contact with the thermocouple, rendering it useless.
It would therefore be beneficial to have a means to increase the lifespan of thermocouples used in a gasification process.